Nanoimprint lithography (NIL) is a forming method in which nanostructures are formed by a punch in curable materials, for example resist. In doing so not only is forming of a large number of nanostructure systems possible, but also the production of high-precision nanostructures on large areas by a step-and-repeat or a roll method. High-resolution surface structuring can be carried out with it. Fundamentally it is distinguished between thermal NIL (hot-embossing NIL) and UV-based NIL methods.
Based on the viscosity of the photoresist, capillary action can completely fill the intermediate spaces of the punch with it. In UV-NIL the punch at room temperature is pressed into the flowable resist, while in thermal NIL methods the thermoplastic resist must be pressed into the resist at elevated temperature (above the glass transition temperature). The punch is removed only after the cooling. In UV-NIL lower contact pressures can be used and the process can take place at room temperature. The UV resist cross-links when exposed to UV radiation into a stable polymer (curing). Therefore structuring can be carried out with “soft” polymer punches and with hard punches. The soft UV-NIL with polymers as the punch materials, depending on the application, is an often economical alternative to structuring with hard punches. Soft UV-NIL (therefore with polymer punches) is also carried out with hard polymer punches. The modulus of elasticity of quartz is roughly 100 GPa. In comparison, the moduli of elasticity of polymers (hard and soft polymers) are up to several orders of magnitude smaller, therefore they are called “soft” compared to quartz (soft lithography).
The most important parameters in the NIL methods are the temperature (mainly in hot-embossing NIL), the contact pressure and the adhesion between the resist and punch. In order to reduce high adhesion between the resist and punch, the punch surface should have a surface energy as low as possible—in interplay with the resist.
Depending on the application, the 3-D structured resist itself can be used as a functional unit or is used as a mask for following etching steps.
For large areas it is difficult to distribute the pressure uniformly over the entire contact surface and to compensate for irregularities. Therefore nonuniform structuring can occur. In order to carry out the structuring of larger areas, embossing is done with a roller or alternatively the entire surface is gradually structured with a smaller punch by shifting the punch (step-and-repeat method).
Nanoimprint is used to produce multilayer structures and (economical) nanostructures (for example integrated circuits in silicon technology) with a resolution below the diffraction limit of light. A large-area nano-embossing process on the entire wafer allows costs, effort and time consumption to be kept low.
Embossing defects which can occur in NIL are for example cracks, nonuniformly filled punch structures (therefore for example air inclusions) and a nonuniform resist layer thickness. The adhesion between the resist and punch is critical. Otherwise distortions or cracks occur. Soft and also hard punches can deform by the applied pressure during the NIL process. Furthermore (dirt) particles are very critical. Particles which are located between for example the resist and punch lead to defects in the entire periphery of the particle.
High-resolution structuring in the low nm range (≤50 nm) is one of the most important advantages of NIL. The replication of structures in the sub 20 nm range is however still a challenge.
Often, for larger wafers, several embossing steps in succession must be carried out to achieve the desired dimensional accuracy. The problem with these embodiments is however ensuring an exact alignment of the many embossing steps with the punch. Generally alignment marks which are located on the substrate and/or on the punch are used. High-precision, aligned embossing of different layers on top of one another is not possible or is possible only with great effort.